Many waste effluents, such as digestate from wet anaerobic digestion of biowaste, contain significant amount of plant nutrients, such as ammonium, as well as organic matter. The organic component in these effluents, often rich in phosphorus and potassium, when properly dried and pelletized, can be used as high-valued bio-fertilizer that is easily transported and applied. Other effluents may contain other contaminants, such as chemicals in the tailings of a mining or oil sands process.
In the case of biowaste streams, currently dominant, wet-digestion technology requires biowastes to be in a low solid content slurry form, achieved by addition of large volume of water, for efficient biogas production and system operation. Ammonia concentration in biogas digestate can reach as high as 2000 ppm, often an important limiting factor for the anaerobic digestion process. The high ammonia concentration in digestate prevents the recycling of process water in the digesters, which could significantly reduce the amount of water being used, as well as the volume of digestate that needs final processing and disposal.
The large volumes of digestate, which could be a significant source of fugitive GHG emissions needs further treatment or disposal. In most cases the volume of digestate produced exceeds the original volume of biowaste entering the digester, and the digestate can be more difficult to treat or dispose than the initial biowaste. Full realization of biogas technology's economic and environmental benefits requires effective treatment of digestate.
The most common treatment option for digestate is storage in lagoons followed by land application. It is bulky, heavy, and relatively low in nutrient concentration, making its application non-economical at any significant distance from the source. Localized over application creates environmental concerns of surface and ground water contamination. Waste water treatment sludge is also often applied on land as fertilizer. Most waste water treatment plants eliminate nitrogen in waste water by denitrification, which converts most of ammonia to nitrogen gas. This process, however, significantly reduces the nutrient of the produced sludge.